COMMENTS ON THE ORIGIN AND EVOLUTION OF SEX 345 



Division of a haploid nucleus giving rise to one migratory "male" 

 and one stationary "female" pronucleus has long been known for 

 the third division in the gametogenesis of ciliates, but apparently the 

 mechanism described by Cleveland has not been recognized in such 

 cases^ although it might apply. However, Sonneborn (1951) has sug- 

 gested that, since these pronuclei are presumably genetically equiva- 

 lent, local cytoplasmic differences would constitute the sex-determin- 

 ing factor. Maupas (1889) and Diller (1936) had earher suggested 

 cytoplasmic determination of the differential behavior among the 

 nuclei derived from the original micronuclei of conjugants and ani- 

 mals undergoing autogamy. 



In many higher animals the sexes differ in the chromosomal con- 

 tent of their cells, the so-called X and Y chromosomes apparently 

 being the differentiating feature. These differentiating chromosomes 

 and the evidence for random segregation of chromosomes on the 

 mitotic spindle, especially in meiotic divisions, would militate against 

 the general occurrence of the "mother-daughter" type of segregation. 

 Thus the evidence indicates that in most cases of chromosome dupli- 

 cation the products are identical, while under certain conditions the 

 two groups of daughter chromatids may be different. Here, surely, 

 is an intriguing field for further research. 



As presented in this volume, "sex" is seen to have a very wide 

 distribution amonff microorganisms. On the other hand, in certain 

 groups it is very uncommon or apparently absent altogether. Its 

 evolution is as unclear as that of many of the groups of organisms 

 themselves. Continued research, both extensive and intensive, may 

 be expected, in time, to find answers to many of the problems that 

 remain to be solved. 



REFERENCES 



Belaf, K. 1923. Untersuchungen an Acfinophrys sol Ehrenberg. I. Die Morpho- 

 logic des Formwechsels. Arch. Protistenk., 46, 1-96. 



. 1926. Der For-ynzvechsel der Frotiste7ikerne. G. Fischer, Jena. 



Cantino, E. C. 1950. Nutrition and phylogeny in the water molds. Qvart. Rev. 

 Biol, 25, 269-277. 



Cleveland, L. R. 1951. Hormone-induced sexual cycles of flagellates. VII. One- 

 division meiosis and autogamy without cell division in Urinympha. J. 

 MorphoL, 88, 385-440. 



Dangeard, P. A. 1910. Etudes sur le developpement des organisms inferieurs. 

 Le Botaniste, ser. 11, 1-311. 



